ewwhite has covered the hardware side of things nicely in their answer.
I'll just add that ESX 4.1 supports Windows 2008r2 just fine (earlier ESX systems probably do too, I just don't keep track of systems I no longer run). We run Windows 2008r2 standard and enterprise on ESX ourselves and we have had no problems at all, in fact it runs very nicely.
This is confirmed in the VMWare Guest OS Guide here but I've included the revelant extract below:
Windows Server 2008 R2
You can install the Windows Server 2008 R2 in a virtual machine using the Windows Server 2008 R2
Before you begin, verify that the following tasks are complete:
- Read “General Installation Instructions for All VMware Products” on page
- Create and configure a new virtual machine.
- Virtual machine. 512MB or more of RAM.
- Host computer. 512MB or more of RAM.
The hard drive for the virtual machine must be 32GB or larger. See Microsoft recommendations: http://www.microsoft.com/windowsserver2008/en/us/system‐requirements.aspx.
- Insert the Windows Server 2008 R2 CD in the CD‐ROM drive.
- Power on the virtual machine to start installing Windows Server 2008 R2.
- Follow the prompts to complete the installation.
- Install VMware Tools.